1. Technical Field
This invention relates to a process for the cooling of gaseous chlorine from elevated temperatures, particularly to the cooling of gaseous chlorine which has been produced electrolytically.
2. BACKGROUND INFORMATION
Chlorine may be produced by the electrolysis of a number of different electrolytes, for example by the electrolysis of an aqueous solution of hydrogen chloride. However, it is generally produced by the electrolysis of an aqueous solution of an alkali metal chloride, particularly sodium chloride, and it is produced on a vast scale throughout the world by the electrolysis of such a solution.
The production of chlorine by the electrolysis of aqueous alkali metal chloride solution may be effected in a variety of different types of electrolytic cells. Thus, the electrolysis may be effected in a cell equipped with graphite or metal anodes and a flowing mercury cathode, chlorine being evolved at the anodes and the sodium which is liberated in the electrolysis reacting with the mercury cathode to form an alkali metal amalgam which is subsequently reacted with water to form an alkali metal hydroxide solution and hydrogen.
Where aqueous alkali metal chloride solution is electrolyzed in an electrolytic cell of the diaphragm type, the solution is charged to the anode compartments of the cell, chlorine which is produced in the electrolysis is removed from the anode compartments of the cell, the alkali metal chloride solution passes through the diaphragms, and hydrogen and alkali metal hydroxide produced by electrolysis are removed from the cathode compartments; the alkali metal hydroxide being removed in the form of an aqueous solution of alkali metal chloride and alkali metal hydroxide.
Where an aqueous alkali metal chloride solution is electrolyzed in an electrolytic cell of the membrane type the solution is charged to the anode compartments of the cell, and chlorine produced in the electrolysis and depleted alkali metal chloride solution are removed from the anode compartments, alkali metal ions are transported across the membranes to the cathode compartments of the cell to which water of dilute alkali metal hydroxide solution may be charged, and hydrogen and alkali metal hydroxide solution produced by the reaction of alkali metal ions with water are removed from the cathode compartments of the cell.
All such electrolyses are operated at elevated temperatures, for example, at a temperature near to the boiling point of the electrolyte, eg at a temperature of the order of 90.degree. C. or even higher, as the efficiency of the electrolysis is greater at such elevated temperatures. The products of electrolysis are necessarily produced at the same temperature at which the electrolysis is effected, and it is necessary to cool the products prior to storage and use. In particular, it is necessary to cool the gaseous products of electrolysis. In the case of gaseous chlorine, cooling is necessary in order than the chlorine may be liquefied by compression, and it has been the practice hitherto to cool electrolytically produced gaseous chlorine by directly contacting the chlorine with water in a packed tower or scrubber, although cooling may be effected by indirectly contacting the chlorine with water.
Cooling of gaseous chlorine by directly contacting the chlorine with water requires the provision of large amounts of cooling water, and furthermore, direct contact between gaseous chlorine and cooling water results in the gaseous chlorine becoming saturated with water vapor at a pressure equivalent to the water vapor pressure at the temperature of the cooling water. The gaseous chlorine may also be mixed with liquid water in the form of a mist. It is necessary to remove this water vapor and mist of liquid water from the gaseous chlorine, particularly if the chlorine is to be liquefied by compression, and it has been normal practice hitherto to remove liquid water in the form of a mist by passing the chlorine through a filter, e.g. a fiber filter, and to remove water vapor by contacting the chlorine with a drying medium, eg sulphuric acid. A large amount of such a drying medium is required in order to dry the. gaseous chlorine.
In GB Patent 1 125 453, there is described a process in which;gaseous chlorine which has been produced electrolytically is cooled by heat exchange with the electrolyte before the electrolyte is charged to the electrolytic cell. The electrolyte which is charged to the electrolytic cell may be an aqueous alkali metal chloride solution, and the heat exchange may be effected by directly contacting the gaseous chlorine with the electrolyte. By operating such a process, the amount of cooling liquid which is required is much reduced; indeed, the need for cooling water may be eliminated, and furthermore, the heat exchange results in the electrolyte being at least partially heated to the required temperature, thus, eliminating the need for at least some of the heating which has been required hitherto.
Direct contact between gaseous chlorine and the electrolyte which is to be charged to the electrolytic cell also provides a further benefit. As the vapor pressure of water over an aqueous solution, such as the electrolyte, is lower than the vapor pressure over water per se at a given temperature the amount of water vapor present in gaseous chlorine which has been cooled by direct contact with such a solution is lower than the amount of water vapor present in gaseous chlorine which has been contacted with water itself. Consequently, the amount of water which must be removed from the cooled gaseous chlorine by drying is correspondingly reduced, and the amount of drying medium which is required is also reduced.
Although chlorine is generally produced electrolytically at a pressure slightly below atmospheric pressure in recent years, proposals have been made to produce chlorine at an elevated pressure. For example, in GB Patent 1 547 062, there is described a process for the electrolysis of an aqueous alkali metal halide solution in a membrane cell in which the pressure in the cell is maintained at a value greater than atmospheric but not greater than 7kg cm.sup.-2 (absolute). Production of chlorine at an elevated pressure is advantageous in that the need for some at least of the compression of the chlorine which is subsequently required in order to liquefy the chlorine prior to storage or use is eliminated.